Hildegard von Mariendorf is one of the principal characters in the series. She served as the New Galactic Fleet's Chief Advisor

She later reigned with Reinhard von Lohengramm when she became his wife and Empress in U.C. 801.

Hildegard, usually referred to by the nickname Hilda, was born in Galactic year 777 into an distinguished noble family of the Goldenbaum dynasty, the sole daughter of the benevolent and just Count Franz von Mariendorf. From an early age, she was distinguished from the other girls of the aristocrat society by her independent ideas. Completely unconcerned, Hilda pursued her own interests in reading, politics and history. Her mother having died when Hilda was young, her open-minded father respected Hilda's unique ideas and choices in life.

In U.C. 797, the old galactic empire spilt into civil war, where the old nobility stood against the new emerging camp of military elites lead by Reinhard von Lohengramm. Hilda convinced her father to plea allegiance to the latter, whereupon she met Reinhard for the first time and impressed him with the clarity of her insight. When the Lohengramm camp emerged victorious from the civil war, Reinhard gained complete political and military power in the empire, and Hilda became Reinhard's chief secretary and assisted him in the progressive reforms that swept across the empire.

From U.C. 798 to U.C. 800, Hilda played an active role on the galactic stage, advising Reinhard wisely on imperial policies and tactics, and became one of his most valued and trusted staff. During Operation Ragnarök, where the empire battled the Alliance, Hilda served as a staff officer and became the first female to board the Flagship Brünhild.

During the critical battle of Vermillion, she perceived that the situation was in Reinhard's disfavour, and persuaded Generals Mittermeyer and Reuentahl to mobilize forces against the Alliance capital, turning the war to Reinhard's tactical victory and saving his life. General Mittermeyer later praised Hilda that "her intelligence is worth more than a fleet." Her role gradually became more public after Reinhard's coronation as the first Emperor of the new Lohengramm Dynasty.

In the year U.C. 800, in the Ninth Battle of Iserlohn, Hilda succeeded General Steinmetz to become the Fleet's Chief Advisor, formally capable of representing the Emperor and command on his behalf on the battlefield. Conscious of her military position, Hilda withheld her political role and refused to advise Reinhard on political matters, despite his repeated requests for her counsel.

In August of U.C. 800, Reinhard was attacked by an assassin who sought vengeance for Reinhard's inaction in the Westland tragedy. Overcame with regret and emotional agony, Reinhard withdrew into his own quarters and Hilda entered to console him. Subconsciously realizing that his reliance on her has deepened past their professional relationship, he asked her to stay, and she agreed, determined to help him to the best of her abilities. The morning after, Reinhard came to Hilda's home and proposed marriage, and Hilda did not immediately reply for the uncertainty whether she can truly give him happiness. She also did not reveal her pregnancy to him until after the Urvashi affair and Reuentahl's subsequent rebellion, which deepened the insight of both Hilda and Reinhard toward their own feelings. When peace was restored, Hilda accepted Reinhard's second proposal, and they were married on January 19, U.C. 801. The officials of the empire greeted their new Empress with enthusiasm and joy. On May the 14th, while Reinhard was away at war, Hilda give birth to a son, whom Reinhard named Alexander Siegfried von Lohengramm. Reinhard's illness seriously exacerbated a mere sixteen days later and was diagnosed with a rare form mutative connective tissue disease, with no know cure. Despite the illness, Reinhard decided to return to the imperial capital for the sake of the people awaiting him there. On July the 18th, Reinhard and Hilda were re-united. On July the 26th, the dying Reinhard said his final words to Hilda: "My Empress, you shall rule the universe more wisely than I. If you think setting up a constitution is the better way then so be it. The universe would be fine as long as the strongest and wisest among all the living governs it. If Alexander Seigfried lacks that ability, there is no need for Lohengramm dynasty to continue. Do everything as you wish; that is my greatest wish..." He died at the age of twenty-five, with their son succeeding him and Hilda ruling as the Empress dowager. She was remembered by later historians as the person who nurtured the Lohengramm Empire to maturity.